This invention relates to the polymerization of ethylene by free-radical initiation at high temperature and pressure in a stirred autoclave by continuous process.
Such process has generally been known and practiced commercially for many years. It has been described in literature and patents, including "Polythene -- The Technology and Uses of Ethylene Polymers" Edited by A. Renfrew and Phillip Morgan and published by Interscience Publishers, Inc., Second Edition, 1960, (especially pages 11-15) and "Encyclopedia of Polymer Science and Technology" Vol. 6, published (1967) by Interscience Publishers, Inc., section entitled "Ethylene Polymers" pages 275 et seq., especially under the heading "Polymerization . . . High-Pressure, Free-Radical Processes", pages 278-281.
In general, operation of the stirred autoclave continuous process involves continuously feeding to the autoclave streams comprising ethylene and free-radical initiator and optionally polymerizable comonomer, diluent, chain transfer agent, telogen or other additive. The temperature of the feed materials must be raised in the reaction zone to the "light-off" or activation temperature, which depends somewhat on the kind of free-radical initiator selected, whereupon polymerization commences. The polymerization reaction of ethylene is strongly exothermic (heat of reaction about 800s-850 cal/g); if no heat loss occurs, the temperature of the reaction mixture rises about 12-13 Centigrade degrees for each one percent of ethylene polymerized. Moreover, as the temperature rises, the activation of the initiator increases and the rate of polymerization increases. The prospect for rapid runaway of the reaction with explosive potential is high. In the usual operation of stirred autoclaves, the reaction is controlled in part by carrying out the process in a continuous manner and selecting the feed materials, temperatures and rates of feed thereof, and rate of flow of reaction mixture through the autoclave (residence time) such that conversion of ethylene to polymer is in the order of about 12 to 14 percent, thereby limiting the temperature rise.
It may be hypothesized that, if some of the heat of reaction could be withdrawn from the reaction mixture in the reaction zone, a larger conversion could be obtained within a given temperature rise (or a lesser temperature rise be experienced for a given conversion). However, in the case of autoclave polymerization of ethylene, it has been thought that heat removal was impractical. The very high pressures in the range of 15,000 to 20,000 psi (1055 to 1406 kg/sq. cm) or more require very heavy, thick sidewalls, in the case of commercial sized autoclaves, walls of steel several inches thick. Heat flow through such thick walls is negligible. Moreover, it was also hypothesized that, if an autoclave sidewall or internal part were drastically chilled in attempt to increase rate of heat transfer from the exothermic reaction mixture, the reaction mixture in immediate contact with such chilled surface could congeal, form an insulating layer and frustrate the cooling; also that the layer of reaction mixture proximate the chilled surface could be cooled below optimum reaction temperature.
In the case of low pressure polymerization of ethylene, e.g. in solution in hydrocarbon liquid medium with catalysts such as activated chromium oxide deposited on porous silica-alumina, there is described in U.S. Pat. No. 3,681,308 reactor apparatus having heat exchange (cooling) surfaces, but the apparatus is not adapted for high pressure, free-radical initiated polymerization of ethylene.
It is an object of this invention to provide means in the art of polymerizing ethylene at high temperatures and pressures with free-radical initiation in a stirred autoclave whereby to remove from the reaction mixture in the autoclave part of the exothermic heat of reaction by indirect heat transfer to a heat exchange fluid.
Other objects and advantages of the invention will be evident from the description that follows.